[clang] [llvm] [RFC][CodeGen] Add generic target feature checks for intrinsics (PR #201470)

[Shilei Tian via cfe-commits]

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@@ -758,6 +758,14 @@ class TypeInfoGen<list<LLVMType> RetTypes, list<LLVMType> ParamTypes> {
 //  * ParamTypes is a list containing the parameter types expected for the
 //    intrinsic.
 //  * Properties can be set to describe the behavior of the intrinsic.
+//  * TargetFeatures is a target feature expression required by the intrinsic.
+//    The empty string means no target features are required. The expression
+//    uses feature names from the target's subtarget feature table. Comma means
+//    AND, | means OR, and parentheses group expressions. The special value
+//    "$custom" (Intrinsic::CustomTargetFeatures) indicates that support is
+//    decided by a target hook (TargetSubtargetInfo::isIntrinsicSupportedByTarget)
+//    rather than by evaluating a feature expression; use it when support depends
+//    on more than the subtarget features, e.g. a particular overload/mangling.
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shiltian wrote:

> This doesn't feel like it's adding any value to the "target feature" concept. One can easily identify intrinsics by name and verify a call, even without ever saying "$custom" in the TD file. That's basically the current practice.

The difference is, we have a unified place and infrastructure to do this instead of scattering the checks all over the place.

> Even with the new design for a callback, that callback can simply be called for every intrinsic, and internally it can recognize intrinsics that do need an extra check. So why bother saying "$custom"?

Without this special feature `$custom`, there would be no callback and everything would simply be just feature based check. Did I misunderstand your point?

https://github.com/llvm/llvm-project/pull/201470